NATIONAL COMMERCIAL GAS ASSOCIATION 
UTILIZATION OF GAS APPLIANCES 


Part I 

ELEMENTARY PRINCIPLES OF COMBUS¬ 
TION AND UTILIZATION OF ENERGY 


Part U 

UNITS OF LIGHT AND ILLUMINATION 
AND PRINCIPLES OF LIGHT DISTRIBUTION 


Part III 

TABLES AND GLOSSARY 


By 

Robert ffrench Pierce 

MEMBER OF BOARD OF EDUCATIONAL CONTROL AND CHAIRMAN OF COMMITTEE 
ON ILLUMINATION FOR PART 3 COURSE 


THIS PAPER IS SUPPLEMENTARY TO THE COURSE 
AND NOT ONE OP THE REGULAR LESSONS 



COPYRIGHT BY THE NATIONAL COMMERCIAL GAS ASSOCIATION 
































Elementary Principles of Combustion 
and Utilization of Energy 

Robert ffrench Pierce 

O understand the production of heat by combustion, 
it is necessary to gain some knowledge of the sim¬ 
pler principles of chemistry. 

All solids, liquids or gases are made up of one or 
more elements. An element is a substance that 
cannot by any known process be divided or split up into dififer- 
ent substances. Substances which may be broken up into other 
substances of a different nature are termed compounds. 

A compound differs from a mixture in that the latter may 
be separated into its component parts by physical means (filter¬ 
ing, sifting, or the like) and shows by its behavior the nature of 
these components. Thus, a mixture of sand and sugar will have 
a sweet taste, like sugar, a gritty feeling like sand, and particles 
of each may be detected by the microscope, and may be sep¬ 
arated by washing out the sugar, which dissolves in water, 
leaving the insoluble sand behind. 

The compound formed by the union of mercury (a silvery 
liquid) with chlorine (a greenish yellow gas) is a white powder 
(calomel) resembling neither of its components in any way, and 
the most powerful microscope is unable to show the slightest 
evidence of mercury particles. 

A further difference between compounds and mixtures is 
that while mixtures may be composed of any proportions of 
substances, compounds are always formed of certain definite 
proportions. For instance, a mixture of oxygen and hydrogen 
may contain equal parts of each, or one part of oxygen to 
twenty of hydrogen, or any proportion between these, or 
any other proportion. A given compound of oxygen and 
hydrogen will always contain the same proportions. Water, for 
instance, always contains two parts or two volumes of hydrogen 
to one part or volume of oxygen. If two parts of hydrogen be 











2 


UTILIZATION OF GAS APPLIANCE COURSE 


burned in the presence of one and one-half parts of the oxygen, 
the hydrogen will combine with one part of oxygen forming 
water leaving the other one-half part as free oxygen. 

For the purpose of making calculations concerning chemical 
compounds, it is convenient to make use of certain assumptions 
as to the nature of their composition. 

This is necessary because chemistry deals with quantities of 
matter so small that they cannot be seen under the most pow¬ 
erful microscope, nor measured by the most delicate instru¬ 
ments. Since, however, the results of calculations based upon 
these assumptions are found to be in entire accordance with 
the results of actual experiment, these assumptions are just 
as useful and reliable as though they were demonstrated facts. 

The first assumption made for purposes of calculation is that 





























































UTILIZATION OF GAS APPLIANCE COURSE 


3 



every compound or element is composed of extremely small 
particles called molecules, each of which is exactly like every 
other one (in the same element or compound), and these mole¬ 
cules are the smallest portion of matter that can exist by them¬ 
selves (not joined with other portions of matter). Each mole¬ 
cule is made up of atoms which are regarded as the ultimate or 
indivisable portions of elements. Atoms cannot exist alone but 
immediately combine with other similar or different atoms to 
form molecules. 

This may be shown diagrammatically. In Figs, i and 2 the 
large irregular areas represent small particles, the squares rep¬ 
resent molecules, and the circles atoms. 

Fig. I represents a mixture of arsenic and sulphur, the 
arsenic particles, molecules and atoms being designated by As 
and the sulphur by S. 























































4 


UTILIZATION OF GAS APPLIANCE COURSE 


Fig. 2 represents a compound of arsenic and sulphur—arsenic 
trisulphide, designated by the symbol AsgSg, indicating the 
elements entering into its composition and their proportions 
(two parts of arsenic—As, to three of sulphur—S). 

The differences between the mixture and the compound of the 
same elements are apparent. 

In the mixture not all the particles and molecules are alike. 
There are particles of arsenic and particles of sulphur, mole¬ 
cules of arsenic and molecules of sulphur. In the compound, 
however, all the particles and all the molecules are alike. That 
is, the mixture of arsenic and sulphur contains particles of ar¬ 
senic and particles of sulphur, while the compound contains no 
particles of arsenic or of sulphur, but only particles of arsenic 
trisulphide (AS2S3). 

This representation is only diagrammatic, and intended to 
show the manner in which we may suppose these particles to be 
arranged. The number of molecules in even the smallest visible 
particle of matter may run into the millions. For instance, one 
cubic inch of air contains 422,000,000,000,000,000,000 (422 
million, million, million) molecules. (The counting of mole¬ 
cules is done by mathematical means by which we are able to 
count the number of these exceedingly small bodies to the same 
degree of accuracy as we may take a census of population. 


® ®(h) 

® © 

CH4. 


®®(h) 

© © 
CH4 



©©© 


©©© 



© © 

CH4 


CH4 


Atom of Hydrogen 

Atom of Cordon 
Molecule of Methane 


FigS. 






UTILIZATION OF GAS APPLIANCE COURSE 


5 




Fig. 4 

To understand how chemical changes such as take place in 
the burning of gas produce heat, it is necessary to consider each 
atom as possessing within itself an attractive force for other 
atoms, and this is of a different degree for different kinds of 
atoms. For instance, under certain conditions, the attraction 
between hydrogen and oxygen atoms is greater than that be¬ 
tween carbon and oxygen atoms. 

This chemical attraction produces the tendency for atoms to 
combine and form different chemical compounds. The man¬ 
ner in which chemical compounds are formed may be illus¬ 
trated by considering the combustion of methane (CH^—one 
part carbon and four parts hydrogen). This gas is the princi¬ 
pal constituent of natural gas of which it composes from 90 to 
98 per cent., and is an important constituent of coal and of car- 
buretted water gas. It is diagrammatically represented in Fig. 
3, in which the squares represent molecules of the gas, and the 
circles atoms of its constituent elements. 

When a sufficient degree of heat is applied to raise this gas 
to the ignition temperature, or burning point, the heat, which 
tends to expand and drive apart the atoms, finally breaks up the 
molecular structure, and for an instant the molecules may be re¬ 
garded as broken up, the atoms being free, that is, not combined 
into molecules. Since, however, atoms cannot exist in a free 







































6 


UTILIZATION OF GAS APPLIANCE COURSE 


State (the molecule being the smallest particle of matter that 
can exist in this state), the atoms instantly seize upon each other 
to form new combinations. Thus the carbon atoms combine to 
form carbon molecules, while the hydrogen atoms form mole¬ 
cules of hydrogen. The carbon molecules gather together into 
particles, and these are raised to incandescence (become light- 
giznng) by the heat of the flame, and produce the luminosity 
which characterizes the open gas flame. Both carbon and 
hydrogen pass rapidly to the outer surface of the flame, where 
the hydrogen combines with the oxygen of the air (Fig. 4), 
forming water vapor (HgO), while the carbon combining with 
the oxygen forms carbon dioxide CO2 (Fig. 5). 

If, however, a plentiful supply of oxygen is well mixed with 
the methane before combustion takes place (as in a Bunsen 
flame) the combining of the carbon and the hydrogen with 
oxygen is practically instantaneous and the formation of car¬ 
bon particles does not take place, hence, the lack of luminosity 
in the Bunsen flame. 

The relation of heat to these chemical changes may be illus¬ 
trated by the well-known fact that the striking together of bod¬ 
ies produces heat. For instance, the repeated blows of a ham- 


Atom of Oxygen /tfo/ftof 

Atom of Cartpon /fycf/o^er? 


'®6® 

COz 


/© 

/HzO j 


/}/o/7? of L 



© © 

© 


© 

COi 


H 2 O 


riG. 5. 






UTILIZATION OF GAS APPLIANCE COURSE 


7 


mer upon a piece of iron will, if sufficiently vigorous and rapid, 
in time produce enough heat to raise the iron to red heat. On 
the other hand, heat is required to expand or separate bodies. 
To drive molecules of water apart and expand it into steam, it 
is necessary to supply heat to the water. In a similar way the 
tearing apart or breaking up of a molecule requires heat to be 
furnished to it, while the rushing together of molecules to form 
compounds produces heat. 

In order to raise methane to the burning or ignition point (the 
temperature at which its atoms of carbon and hydrogen will be 
driven apart) a certain amount of heat is required, hence the 
necessity of the lighted match, electric spark, glowing platinum 
or pilot flame. After the molecules are broken up, however, 
their atoms combine anew with the oxygen of the air, and the 
rushing together of the atoms to form these new combinations 
produces sufficient heat to break up the incoming methane, and 
thus keep up the combustion, in addition to producing heat for 
useful purposes. 

Thus there are two kinds of chemical reactions—heat-pro¬ 
ducing, called exothermic, and heat-requiring or heat-absorb¬ 
ing, called endothermic. 

The heat produced by exothermic reactions is the result of 
the rushing together of the atoms of the gas during their con¬ 
version into nev/ compounds. It should be borne in mind 
that the distinction between endothermic and exothermic re¬ 
actions does not depend upon whether a given compound is 
formed or broken up, but upon whether the reaction produces 
heat or absorbs heat. The breaking up of methane into carbon 
and hydrogen, and the forming of calcium carbide by the 
union of calcium (lime) and carbon in the electric arc are 
both endothermic reactions, requiring heat to produce them. 
On the other hand the formation of water vapor by the union 
of hydrogen and oxygen is an exothermic reaction producing 
heat. In the gas flame, both endothermic and exothermic 
reactions are going on at the same time and it is the excess 
of heat produced by the exothermic reactions over that required 
for the endothermic reactions that is available for doing work. 


8 


UTILIZATION OF GAS APPLIANCE COURSE 


A combustible gas is therefore a carrier of energy, which may 
under proper circumstances be converted into heat. 

For this reason, the quality of a gas (as far as usefulness for 
producing heat is concerned) is best expressed by the amount 
of heat which may be extracted from it. 

Heat is entirely different from temperature. Adding heat 
to a body does not always increase its temperature, though 
the unit by which heat is measured (the British Thermal Unit 
or B. T. U.) is defined as the quantity of heat which will in¬ 
crease the temperature one pound of water one degree Fahren¬ 
heit from 62 degrees (abbreviated 62°). But while one B. T. U. 
will raise the temperature of one pound of water from 62° 
to 63° Fahr., nearly 1,000 B. T. U.’s may be added to a pound 
of water at 212° Fahr. without raising its temperature in the 
leavSt. This is because at 212° Fahr. water at atmospheric 
pressure turns into steam at the same temperature, and this 
conversion uses up a large amount of heat. This heat is called 
“latent heat of vaporization.” Furthermore, different sub¬ 
stances show different increases in temperature when supplied 
with the same quantity of heat, and with each substance the 
increase may vary according to the temperature. The quan¬ 
tity of heat required to raise one pound of any substance one 
degree Fahr. from any particular temperature is called the Spe¬ 
cific Heat of the substance at that temperature. 

Substances differ not only with regard to their capacity for 
absorbing heat, but also in the readiness with which they trans¬ 
mit or conduct heat, and in order to design gas appliances, or to 
select or use them intelligently it is necessary to take all these 
things into consideration. 

The principal consideration in the design and operation of 
gas-burning appliances is the production of the highest prac¬ 
ticable temperature in the flame itself. This is particularly true 
of incandescent gas lamps, and is of very great importance in 
nearly all gas-burhing appliances. 

The flame temperature' determines the efficiency of the ap¬ 
pliance, other things being equal. For instance, suppose we 
• wish to maihtain a melting-pot at a temperature of 500° Fahr. 


UTILIZATION OF GAS APPLIANCE COURSE 


9 


Since heat cannot flow except from a body of higher to one or 
lower temperature, all the heat in the gases below 500° is 
wasted, while that above 500° is practically all available for 
maintaining the temperature in the pot. Now, if the tempera¬ 
ture of the flame be 1,500°, one-third of the heat represents 



temperatures below that of the metal, and hence useless, while 
two-thirds of the heat is available or useful. If, however, we 
increase the temperature of the flame to 2,500°, then only one- 
fifth is wasted, while four-fifths is useful. This is illustrated 
diagrammatically in Fig. 6. 

Just as water can flow only from a higher-to a lower level, 














10 


UTILIZATION OF GAS APPLIANCE COURSE 


heat can flow only from a body of a higher to one of a lower 
temperature. If water is to be discharged at a 500-foot level, 
only sources above that level are available unless mechanical 
means, such as pumping, are utilized. If it is desired to add 
heat to a body already at 500 degrees, it must be brought in 
contact with substances of higher temperature. As soon as 
these substances are cooled to 500 degrees by contact with the 
heated body, they must be rejected and the heat in them is 
wasted as far as this particular process is concerned. 

Thus, the efflciency of an appliance depends to a great extent 
upon providing proper conditions of combustion for the main¬ 
tenance of high flame temperatures. For each adjustment of a 
Bunsen burner there is one gas pressure and gas composition 
at which the highest flame temperature will be reached. If this 
is disregarded and variations of pressure are permitted at the 
customer’s burners, and if gas of varying composition be sup¬ 
plied, even though the candle-power and heating value (B. T. 
U.) of the gas remain the same, loss in efficiency will occur. 

Table I shows the variations in gas consumption and in 
light-giving power (which is an indication of temperature) in 
an inverted incandescent gas lamp. In this table Column i 
gives (in inches of water column) the limits between which 
the gas pressure varied. Column 2 gives the increase in con¬ 
sumption resulting from raising to the upper limit the pressure 
upon a lamp adjusted for the lower limit. Column 3 gives the 
decrease in light output resulting from dropping to the lower 
limit the pressure upon a lamp adjusted for the upper limit. 


Table I 


Col. I 


Col 2 


Col. ? 

13% 


2 — 
2^—3 


13% 

9 

8 

7 

6 

6 


II 


3 —354 
3/2—4 

4 —4/ 
4/2—5 


6 

8 

3 

6 


UTILIZATION OF GAS APPLIANCE COURSE 


II 


Col I 

Col. 2 

Col. 3 

' —3 

22 

20 

> —4 

17 

10 

■ —5 

14 

II 

' —4 

48 

26 

1 —5 

30 

13 

! — 3>4 

34 

24 

; —434 

24 

18 


It is the occasional variations in the composition of the gas 
that are responsible for the housewife’s complaint that the gas 
is “poor” on certain days. It may be entirely up to the legal 
standard as to candle-power and heating value, but if it is 
of different composition from the gas on which the burner was 
adjusted, it will fail to do efficient work in the lamp, oven or 
water-heater. 

The temperature of the open flame is necessarily low. The 
flame tends to spread or extend out until it comes in contact 
with enough oxygen to consume the carbon and hydrogen in 
the gases. This produces a flame having a large surface. 

Just as water will flow more rapidly through a large pipe 
than through a small one, so heat will escape more rapidly 
from a large surface than from a small one. As the tempera¬ 
ture of the flame is due to the heat in its gases, the more rap¬ 
idly this heat is lost the lower the temperature of the flame. 

The smaller the flame in which a certain amount of gas 
may be burned the greater the flame temperature, and to reduce 
the volume or size of the flame, it is necessary to furnish oxy¬ 
gen more rapidly at the place where combustion is taking place, 
so that it will not be necessary for the flame to spread out so 
far in order to encounter enough oxygen to consume its gases. 

In the Bunsen burner, which is, broadly speaking, the type 
used on every modern gas burning appliance (except reflector- 
type room heaters), the gas issues in a thin stream from a small 
orifice (Fig. 7) surrounding which are air-ports—through 
which the primary air required for the combustion of the gas 
is drawn by the velocity of the gas stream. The air so drawn 
in or entrained is called primary air because it is the first air to 


12 


UTILIZATION OF GAS APPLIANCE COURSE 


be mixed with the gas. The balance of the air required for 
combustion is obtained from the air surrounding the flame and 
is termed ''secondary air” 

^6 Ref/ey. J^arTip 



FigR 



























UTILIZATION OF GAS APPLIANCE COURSE 


13 


The action of the gas in entraining primary air depends upon 
the friction of the gas-stream against the surrounding air. The 
particles of air next to the gas-stream are swept along with it 
into the mixing chamber. Other particles rush in to take the 
empty places, and thus a steady flow of air through the air¬ 
ports, along the gas-stream and into the mixing chamber is se¬ 
cured. The gas and air mix more or less thoroughly in the 
mixing chamber—and the mixture is ignited at the nozzle or 
burner tip. There being nearly or quite enough oxygen in the 
mixture to consume the gas, combustion takes place rapidly in 
a very small space, and a high temperature is produced. 

If the amount of oxygen (or air) be less than that necessary 
for complete combustion, the balance (called secondary air) 
is taken from the air surrounding the flame. If the mixture is 
greatly deficient in oxygen the unconsumed gases will burn 
outside the conical flame—in much the same manner as the gas 
from an open flame tip, and with the same yellow luminosity. 

The presence of this yellow portion in a Bunsen flame is 
therefore evidence that insufficient air is being drawn in or 
entrained through the air-ports. 

The size of the flame for a given volume of gas consumed 
will depend upon the velocity of the outflowing mixture and 
the quantity of air present in the mixture. 

There are limits, however, to the increases in flame tempera¬ 
ture that may be obtained by increased entrainment of air. 
Wherever a mixture of gas and air flows into a flame there is 
a tendency for the flame to travel back along the outflowing 
stream of this mixture. This tendency is opposed by the ve¬ 
locity of the mixture itself, and when a balance is reached the 
flame remains stationary. 

The speed with which the flame will travel through a still 
mixture of air and gas depends upon the proportions of gas and 
air present. The greater the proportion of air (up to that re¬ 
quired completely to consume the gas) the greater the speed of 
the flame. 

At the low pressures ordinarily in use it is impossible to en¬ 
train all the air necessary for combustion because the speed 


14 


UTILIZATION OF GAS APPLIANCE COURSE 


with which the flame travels back toward the orifice is greater 
than the velocity of the issuing mixture. At higher gas pres¬ 
sures, however, the velocity of the mixture is increased, and 
at certain pressures a sufficient velocity may be secured to per¬ 
mit the entrainment at the orifice of all the air required for 
combustion, without danger of the flame traveling back to the 
orifice. Also a more perfect and intimate mixture of the air 
and gas is secured due to the high velocity and consequently 
violent agitation at the point of mixture. Since the temperature 
of a flame (for a given quantity of the gas) depends upon the 
smallness of the flame, and this latter depends upon the pro¬ 
portions of air and gas and the uniformity of the mixture, it is 
evident that the efficiency of a burner will depend (i) upon 
the pressure of the gas at the orifice, (2) upon the pressure 
of the air at the primary air-ports, and (3) upon the design 
of the Bunsen tube (with reference to entraining air and pro¬ 
ducing a perfect uniform mixture). 

In the ordinary atmospheric burners (those in which no air 
under greater than atmospheric pressure is used) the third 
consideration will be of the greatest importance, since the gas 
pressure limits are usually fixed by service conditions. If, how¬ 
ever, the burner be turned down at the key, or the service cock 
at the meter be partially closed, the appliance will not receive 
the full service pressure, the flame temperature will be lowered 
and more gas will be required to perform a given work. 

In particular appliances certain considerations other than the 
production of maximum flame temperature must be taken into 
consideration. These will receive attention under the lessons 
dealing with individual appliances. 

The relations which conditions of gas service bear to the 
performance of appliances will also vary with the particular 
appliance under consideration. The following general state¬ 
ments, however, hold good with reference to all appliances. 

GAS PRESSURES. 

With gas of a given quality the velocity of flow through the 
orifice will depend (i) upon the pressure; (2) upon the form 


UTILIZATION OF GAS APPLIANCE COURSE 


IS 

of the orifice. The higher the pressure, the greater the velocity 
of the issuing jet, and accordingly the greater the volume of 
gas passed during a given time. If the pressure be increased, 
the greater volume of gas passed will require a proportionately 
greater volume of air in order to maintain perfect combus¬ 
tion and the same flame temperature. The increased velocity 
of the jet enables it to entrain a greater quantity of air, but not 
in direct proportion to the increased quantity of gas flowing. 
The quantity of gas issuing is proportional to the velocity; 
that is, if the velocity be doubled, the quantity of gas passed 
will also be doubled. The amount of air entrained, however, 
is proportional to the square of the velocity. That is, if the 
velocity of the issuing gas be doubled (2X2=) 4 times as 
much air will be entrained. If the velocity be tripled (3X3=) 
9 times amount of air will be entrained. This increased 
air results in a tendency for the flame to travel back, 
which is opposed, however, by the increased velocity of the 
mixture of gas and air issuing from the nozzle. If the Bunsen 
tube be so arranged that the increased amount of air has no 
opportunity to become uniformly mixed, a roaring sound will 
be produced. This trouble may be corrected (i) by reducing 
the air supply by partially closing the air-shutter (when pres¬ 
ent) or (2) by interposing gauzes or similar obstructions in 
the mixing chamber. These serve two purposes. In the first 
place by obstructing the mixing chamber, they tend to oppose 
the entrainment of air, and thus reduce the excess. Further¬ 
more, they tend to correct momentary tendencies to “flash 
back” by conducting the heat away from the flame and reducing 
the temperature of the gases below the ignition point. When 
the flame travels back to the gauze at frequent intervals, a 
“popping” sound occurs which is a symptom of excessive pri¬ 
mary air. 

If, on the other hand, the gas pressure at the orifice be re¬ 
duced, less gas will pass and insufficient air will be entrained. 
If the velocity of the gas be reduced to one-half, the amount 
of air entrained will be reduced to one-fourth. This will pro¬ 
duce lower flame temperature, and in some cases incomplete 


1 6 UTILIZATION OF GAS APPLIANCE COURSE 

combustion (evidenced by a yellow flame tip and in incan¬ 
descent lamps carbonized mantles). 

This may be corrected (i) by a wider opening of air-ports; 
(2) by removing gauzes or substituting one of larger mesh in 
the mixing chambers. 

SPECIFIC GRAVITY. 

Specific gravity is a measure of the weight of a gas. It is 
expressed by the ratio between the weight of a certain volume 
of gas and the same volume of air. If one cubic foot of a 
gas weighs one-half as much as one cubic foot of air the 
‘‘gravity” of the gas is said to be one-half or as always ex¬ 
pressed in figures—.5. 

The specific gravity of gases varies according to their chem¬ 
ical constituents and the proportions of the latter. The specific 
gravity of typical coal gas is only about two-thirds that of 
typical carburetted water gas, and the specific gravity of both 
carburetted water and coal gases will vary if the processes of 
manufacture are not uniform, and if means are not taken to 
avoid changes in composition through the “dropping out” of 
illuminants in distribution to the consuming premises. 

The “illuminants” (see Glossary) are heavy vapors which 
are readily carried by the non-illuminating gases at ordi¬ 
nary temperatures and pressures, but less readily at lower tem¬ 
peratures and higher pressures, under which conditions they 
drop out more or less completely. If for any reason the pres¬ 
sure of a gas is increased or its temperature lowered, a portion 
of the illuminants come dov/n as liquids and collect in the pipe, 
and the remaining gas is thus of different composition. 

The effect upon the burner of varying specific gravity is 
somewhat similar to that of varying pressure, and often far 
more pronounced. The lighter the gas (the lower its specific 
gravity) the greater the velocity of the jet issuing from a given 
orifice and the greater the volume of gas passed during a given 
time. As the change in specific gravity is accompanied by a 
change in chemical constitution, the amount of air required for 


UTILIZATION OF GAS APPLIANCE COURSE 1 7 

combustion is also changed. Coal gas requires more air for 
combustion than does carburetted water gas; therefore, if in a 
burner properly adjusted for a certain mixture of coal and 
carburetted water gas, the proportion of coal gas be increased, 
insufficient air will be entrained, and low flame temperature or 
incomplete combustion will ensue. If the proportion of water 
gas be increased, there will be a tendency to ‘‘flash back.” 

It is sometimes thought that gas burners may be so designed 
as to give high efficiencies and at the same time not be sus¬ 
ceptible to reasonably large changes in gas pressure and “grav¬ 
ity.” This is not the case. The most efficient point of operation 
for any burner is just before the “flashing back” point. A 
burner operated at this point will be susceptible to very small 
changes in gas pressure and quality. 

In order that a burner may be able to withstand considerable 
changes in pressure and “gravity” without “flashing back” it is 
necessary to operate it much below the highest efficiency. 

This is somewhat analogous to the case of the incandescent 
electric lamp. The higher the voltage at which it is operated, 
the higher the efficiency of light production, but the greater the 
damage from the momentary application of even a small ex¬ 
cess in voltage. For instance, if a tungsten filament electric 
lamp be designed to operate at .81 candles per watt (1.23 watts 
per candle) on no volts, and to have a useful life of 1,700 
hours when operated at this point, an increase in voltage to 114 
will cause the duty (see p. 75) to increase to .885 candles per 
watt (1.13 watts per candle), but the useful life will be reduced 
to 1,000 hours, or over one-third. 

The most important features in good gas service are uni¬ 
formity of pressure and quality. The candle-power standard 
gives no indication whatever of the value of a gas to the con¬ 
sumer except for use in open-flame burners, which are rapidly 
becoming obsolete. For most purposes high candle-power gas 
is much poorer than low candle-power gas, on account of the 
difficulty of entraining sufficient air to obtain the highest flame 
temperature. 

Neither is the heating (or calorific) value of much impor- 


18 UTILIZATION OF GAS APPLIANCE COURSE 

tance, provided the price at which the gas is sold be commen¬ 
surate. That is, if for a certain purpose 500,000 B. T. U.’s 
are required, and gas of 500 B. T. U.’s per cu. ft. is available 
at $1.00 per thousand cubic feet, or gas of 250 B. T. U.’s at 
$0.50 per thousand, it is immaterial to the user whether he 
purchases 1,000 cu. ft. of 500 B. T. U. gas for $1, or 2,000 cu. 
ft. of 250 B. T. U. gas for $1. The close regulation of gas 
pressure and uniformity of quality is of immensely greater im¬ 
portance than either calorific value or candle-power. 

Having obtained a flame of the highest possible temperature, 
it is necessary next to consider the transferring of the heat 
from the flame to the material to be heated with the least pos¬ 
sible loss. 

Heat may be transferred from one body or place to another 
in three different ways; 

First, by conduction by which heat is conducted through a 
body in much the way that electricity is conducted through a 
wire. Each particle on becoming heated transmits or conducts 
a portion of its heat to adjacent particles, and in this way the 
heat may be said to travel through the body. Different sub¬ 
stances possess different capacities for conducting heat, or dif¬ 
ferent thermal conductivities. 





^ 

Iron 

X JL X JL 1-1-1-1- 

LeocL 

— 1-1 —I—I—T—T—I— 


^666' ' ' ‘i' '666666 


Fig. 8 

Thus, if a bar of iron and a bar of lead of equal size (Fig. 
8) be heated at one end of each, the heat will be more rapidly 
conducted through the iron, and this may be shown by attach¬ 
ing small balls by means of some fusible material which will 
melt at a low temperature. 














UTILIZATION OF GAS APPLIANCE COURSE 


19 


The balls nearest the heated ends will drop first, and the 
successive dropping of the balls will indicate the rapidity with 
which the heat is being conducted by the metal. It will be 
thus seen that the iron is the better conductor. 

Second, by convection or the actual moving of the heated 
body itself. An example of this is seen in the heating of water 
contained in a vessel heated at the bottom by a flame or other 
means (Fig. 9). The heat is transferred by conduction 



Fig. 9. 


through the bottom of the vessel and to the water at “C.’' 
This water, becoming lighter by expansion, rises and cooler 
water from the top and sides of the vessel flows downward 
to take its place. 

This water is heated in turn and the process repeated con¬ 
tinuously, and the heat from the flame is transferred to all 
the contents of the vessel. Thus, the heat from the flame en¬ 
ters the vessel by conduction, and is distributed throughout the 
contents by convection, being carried by the circulating water 
currents. 












20 


UTILIZATION OF GAS APPLIANCE COURSE 


Third, by radiation, in which energy is radiated (or sent 
out in rays) through a non-conducting medium (called the 
“aether”) in the same manner as light. A somewhat similar 
action occurs when a stone is dropped into a pool of water. 
The motion imparted to the water is carried by the waves pro¬ 
duced, throughout the surface of the pool, and energy is thus 
radiated to distant points. In fact, light rays are simply en- 
ergy waves of certain lengths to which the “retina” of the eye 
is sensitive, and rays which are too long to affect the retina 
are often called heat rays. When these “energy waves” en¬ 
counter objects which absorb them, heat is produced, and since 
bodies prodtuce these energy waves by virtue of their tem¬ 
perature, this process furnishes a means of transmitting heat 
from one place to another, even though the intervening space 
be not a conductor of heat. 

The heating of the earth’s atmosphere by the sun involves 
all three methods of heat transferrence. First, the radiant en¬ 
ergy sent out by the incandescent gases of the sun encounters 
the earth, and being absorbed and transformed into heat, the 
earth is warmed. This heat is transferred into the lower layers 
or strata of air by conduction, and these rising upward, make 
place for cooler air and set up a process of conve'ction. The 
currents of air (or winds) so produced thus carry heat from 
one portion of the earth’s surface to another. 

In applying the heat of burning gases for any purpose we 
may make use of any or all of these methods of heat transfer¬ 
rence. 

The most important thing to bear in mind is that the efficient 
use of gas for heating demands the reduction of heat losses to 
the lowest possible point. 

Heat may be lost by conduction, convection or radiation. 
Losses in the first manner are comparatively easy to reduce 
since many substances, notably asbestos, firebrick, etc., conduct 
heat very poorly. Air is also a comparatively poor conduc¬ 
tor, but since, if allowed to move about freely it carries away 
heat through convection, means must be taken to prevent this 
movement. Consequently, when air is used as a heat insulator 


UTILIZATION OF GAS APPLIANCE COURSE 


21 


(or non-conductor) it must be held in small spaces or cells, 
such as are formed by a packing of cork, asbestos, mineral wool 
or the like. The substance used must, of course, be of a sort not 
affected by the temperature to which it will be subjected. The 
quantity of heat transferred by conduction is proportional to 
the time, to the conducting area, to the difference in tempera¬ 
ture between the two points, and to the thermal conductivity 
(heat-carrying ability) of the material. 

During a given time, with the same temperature differences 
between the ends of the bars, an iron bar square (^ sq. in. 
in cross-section) will conduct away four times as much heat as 
one I" square (vVsq. in. in cross-section). 

In the same bar, twice as much heat will flow away under 
a difference of 4° in the temperatures of the ends as would flow 
under 2° difference. Practically the same law holds with ref¬ 
erence to the loss of heat through a sheet of iron in contact 
with the air. 



r iG. 10 






















22 


UTILIZATION OF GAS APPLIANCE COURSE 


Quite similar in principle to the conduction of heat is the 
transmission of water through pipes. At a given pressure or 
*‘head” more water will flow (in a given time) through a two- 
inch than through a one-inch pipe, and twice as much water 
will flow in 10 minutes as in 5 minutes. Through a pipe of 
given size more water will be discharged under high pressure 
than under low pressure, and more through a clean pipe than 
through a foul one, likewise more through a short pipe than 
through a long one. 

The quantity of water delivered in a given time depends 
upon the velocity with which it issues from the pipe, and the 
area of the end of the hole from which it issues. 

The velocity depends upon the “head"’ or pressure of the 
water and the friction which the water encounters in flowing 
through the pipe, which retards the velocity. 

Fig. 10 illustrates the greater velocity (and hence quantity 
of water delivered from a given hole or orifice) due to greater 
pressure or head (corresponding to temperature difference). 





















UTILIZATION OF GAS APPLIANCE COURSE 


23 




Fig. 12 

Fig. II illustrates the greater quantity delivered at a given head 
or pressure (hence at the same velocity) through the larger 
orifice. Fig. 12 illustrates the reduced velocity (hence lesser 
quantity delivered) due to friction in a longer pipe. 

Thus, to reduce heat losses it is necessary to reduce the area 
of heat conducting bodies and surfaces. 

The escape of radiant energy may be practically prevented 
by surrounding the heated body by a reflecting surface, such 
as polished metal, which will reflect a large portion of the heat 
back into the heated body. Surrounding the heated body by 
insulating substances which absorb radiant energy and trans¬ 
form it into heat serve to retain a portion of the energy in 
the apparatus. 

The well-known Thermos bottle (Fig. 13) for keeping liquid 
'either cold or hot uses a vacuum (a perfect non-conductor) as 
insulation against the escape of heat by conduction or convec¬ 
tion, and a mirrored surface to minimize radiation losses. 

The amount of heat radiated from a body is proportional to 
the fourth power of the absolute temperature (491° plus Fah¬ 
renheit temperature), depending upon the material. An increase 
of 10° in the absolute temperature of a body will increase the 
quantity of heat radiated by 46 per cent. The quantity of heat 



















24 


UTILIZATION OF GAS APPLIANCE COURSE 


radiated will also depend upon the condition of the surface, 
being much less from smooth than from rough surfaces, and the 
temperatures of bodies receiving the radiation. This latter con¬ 
sideration is negligible except where the radiating body is at a 
temperature far below that of any gas flame. 

The expedients that will be justified for minimizing heat 
losses in any particular device will depend upon commercial 
factors—the cost of applying or furnishing the means for re¬ 
ducing losses, the durability of the appliance, its weight, size, 
etc., and the cost of the combustible used to produce the heat, 
for obviously it would be foolish to spend more in saving heat 
than the heat itself is worth. 

In very few processes is the mere cost of producing a cer¬ 
tain amount of heat the main consideration. In domestic appli¬ 
ances, cleanliness, comfort and convenience are usually more 
important and more appreciated than mere economy. 

In industrial appliances, on the other hand, the quality of the 



Principle of Thermos Bottle 

riG. 13 

















UTILIZATION OF GAS APPLIANCE COURSE 


25 


product and the rapidity with which it may be turned out are 
likely to be of prime importance. 

In certain special fields, hygienic considerations may be up¬ 
permost. All other things being equal, however, economy is the 
deciding factor, and it is always a point in favor of the appli¬ 
ance possessing it, and the salesman should always take pains 
to install the most economical appliance which will otherwise 
meet the customer’s requirements. 

Unfortunately, it is not easy to obtain comparative data on 
the relative economies of various appliances. This is due to 
the fact that most published reports upon the operation of 
appliances describe their performance with reference to one 
particular application under one condition, and no comparison 
is possible unless the conditions of test are identical, for it is 
only in this way that we determine whether or not the design 
of the appliance itself is responsible for the differences in per¬ 
formance. For instance, suppose we have two tests of steam 
engines, one consuming 12.5 pounds of dry steam per horse¬ 
power hour and one 12 pounds of steam superheated 150°. Un¬ 
less we know the extent to which superheat affects the per¬ 
formance of this particular type of engine, we have no means 
of knowing whether the superheat or the design of the engine 
itself is to be credited with the improved economy. 

Similarly, the statement that one lamp gives 20 candle-power 
for each cubic foot of gas burned while another gives 22 is of 
no use whatever; unless we know that both lamps were fitted 
with identical mantles and burned upon the same gas at the 
same pressure and upon the same day, for all these factors 
affect lamp performance. Even a difference in atmospheric 
humidity (the amount of moisture in the air) might account 
for the entire difference in performance. 

Thus, before we may make the statement that one appliance 
is more efficient than another, we must first make sure that all 
conditions of test are identical in order that any differences in 
performance may properly be credited to the superiority of the 
appliance itself. 

Furthermore, it is desirable to use some system of units for 


26 


UTILIZATION OF GAS APPLIANCE COURSE 


expressing the energy contained in different fuels, etc., so that 
they may be compared with each other. It is not possible to 
make a direct comparison between a pound of coal and a cubic 
foot of gas, but by a third energy unit we may compare the 
energy in a pound of coal with that in a cubic foot of gas. 

Such a unit is the British Thermal Unit before described. 
By means of “Conversion Tables” any unit of energy or work 
may be expressed in B. T. U.’s or vice versa. 

This is possible because heat, being a mode of motion, is 
energy, and energy results in heat. The unit of work is the 
“foot-pound,” the work required to raise one pound one foot, 
regardless of the time required in doing it. The unit of rate 
of work is the Horse-pozver, which is 33,000 foot pounds per 
minute—that is, 33,000 pounds raised one foot in one minute, 
or one pound 33,000 feet in one minute, or 33,000 

-=550 

60 

pounds raised one foot in one second, etc. Thus, one horse¬ 
power expended continuously during one hour, or one H. P. 
hour = 33,000 X 60= 1,980,000 ft. pounds. One B. T. U. = 
778 ft. pounds. Thus, if a mass weighing one pound were 
dropped 778 feet into one pound of water at 62° F., the tem¬ 
perature of the water would be raised one degree. Kilo-watt 
hours, horse-power hours, etc., may always be expressed in 
B. T. U.’s or foot-pounds and vice versa. 

A short table of conversion factors is here given. 

I ft. lb. = .001285 B. T. U. = .0003766 watt-hours = 
.0000003766 k.w. hours = .0000005051 H. P. hours. 

I B. T. U. = 778.1 ft. lbs. = .293 watt-hours = .000393 
H. P. hours = .000293 k.w. hours. 

I Kilo-watt hr. = 2655000 ft. lbs. = 1.34 H. P. hr. = 3413 
B. T. U.’s. 

I Horse-power hr. = 1980000 ft. lbs. = 745.6 watt-hours = 
.7456 k.w. hours = 2544 B. T. U.’s. 

The efficiency of an appliance or machine is the ratio between 
the energy utilized by the appliance and that furnished to it. 



UTILIZATION OF GAS APPLIANCE COURSE 


27 


Efficiency as used in this sense must not be confused with the 
ordinary conversational use of the word in which it signifies 
adaptability to a desired end. It deals with energy units only. 
Efficiency may be measured or determined in two ways, the 
work done by the machine and the energy put into it may be 
measured, and the former divided by the latter, or, the losses 
may be measured and subtracted from the energy put in, the 
result being divided by the energy put in. An example of the 
first method is the determination of the efficiency of a range 
top burner by burning a given quantity of gas in the burner 
(the B. T. U.’s of the gas being known) and measuring the 
increase in temperature of a known weight of water in a ves¬ 
sel above the burner. Suppose the vessel to contain 8 pounds 
of water at 40° F. before the test. After burning one cu. ft. 
of gas under it the temperature rises to 70° F. The rise in 
temperature is 30°. If the gas contains 600 B. T. U.’s (as 
determined by previous experiment) this amount of heat would 
be sufficient to raise one pound of water 600° or 8 pounds 
600 

- = 75° if all the heat were used. Since the actual rise is 

8 

30 

but 30° — =40 per cent, of the heat in the gas has entered 

75 ^ 

the water and the efficiency of the appliance is 40 per cent. 

Sometimes it is more convenient to ascertain the losses than 
to measure the useful zvork. For instance, it is often incon¬ 
venient to place an artificial load on electrical apparatus of high 
capacity in order to ascertain the performance under load. In 
testing large transformers it is often found convenient to supply 
only enough energy to supply the losses and to calculate the 
efficiency from these measurements. If the losses correspond¬ 
ing to full load operation in a 1,000 k.w. transformer are found 

1,000 — 20 

to be 20 k.w., then the efficiency at full load is-= 

1,000 

98 per cent. 



28 


UTILIZATION OF GAS APPLIANCE COURSE 


The combined or over-all efficiency of a combination of ap¬ 
paratus is the product of the efficiencies of the different parts. 
For instance, suppose an installation to be composed of a steam 
boiler having an efficiency of 70 per cent., a steam engine of 20 
per cent, and an engine-driven pump of 95 per cent. For every 
100 heat units in the coal 70 per cent, or 70 heat units will be 
utilized in the boiler. Of these 70 units 20 per cent, or (70 X 
.20=) 14 will be utilized in the engine, and 95 per cent, of 
the 14 or (14 X -95 =) i3-3 will be useful for pumping water. 
Thus, the over-all efficiency is .70 X -20 X -95 == -133 = 13-3 
per cent. 

The word efficiency is sometimes loosely used to signify the 
amount of energy in one form obtained from a given amount 
in another form, such as the number of candle-power given 
by each watt of electricity or cu. ft. of gas consumed per hour. 
Thus, an incandescent electric lamp consuming 64 watts per 
hour and giving 16 horizontal candle-power is said to have 
/16 \ \ 

an efficiency of I — = I *^5 candles per watt, or I — = I 4 
\64 / . ^ 

watts per candle. To express this and similar quantities the 
term duty is preferable, and will be used throughout this course. 
Duty should always be expressed in quantity of product for each 
unit of energy consumed—that is, the duty of an electric lamp 
should be given in candles per watt rather than watts per 
candle. The latter has been most generally used because the 
figures for the incandescent lamp happen to come in whole 



while the former comes as a fraction 


UTILIZATION OF GAS APPLIANCE COURSE 


29 


PART II 

UNITS OF LIGHT AND ILLUMINATION AND LAWS 
OF LIGHT TRANSMISSION 

In passing from the luminous source to the eye, light passes 
through various processes and undergoes various changes all 
of which absorb a certain amount of light and thus affect the 
quantity delivered to the eye as available for producing vision. 
It is therefore necessary to formulate units by which light 
may be measured so that the efficiency of these processes may 
be determined. The utilization of light may be compared to 
the utilization of steam or electricity, and the various processes 
may be divided into the following group: 

I Production. — The light source corresponds to the steam 
boiler or electric generator, where energy is turned into the 
form in which it is to be transmitted. 

H Transmission .—The aether* through which light waves 
are propagated) and the various reflecting surfaces, including 
the lamp equipment and the illuminated objects, correspond 
to the steam piping or electric transmission and distribution 
system, through which the energy is carried to the apparatus 
in which it is to be utilized. 

Ill Utilization.—ThQ eye corresponds to the steam engine, 
electric motor, lamp or heating apparatus, in which the trans- 
Thifted energy is converted into the form in which it is de¬ 
sired to use it—mechanical power in the steam engine, mechan¬ 
ical power, light, or heat in the case of electricity, and the 
sensation of sight in the case of light. 

It is therefore necessary, first of all, to devise some means 

* To explain certain known facts concerning the-nature and behavior 
of light waves, it is necessary to assume the existence of a substahce which 
pervades all matter and spacej even the densest metals r and: the most 
perfect vacuum.. This substanqe possesses-.some, of the characteristic^,, of 
solids and is called the “aether.” (See ahy good encyclopedia 6r text- 
b'pok on physics.) • . : : '^no:^r:'on 



30 


UTILIZATION OF GAS APPLIANCE COURSE 


of measuring the quantity of light produced at the source. 
Since the results produced by light are, broadly speaking, ap¬ 
parent to the eye only, and as a physiological sensation which 
cannot be translated into terms of energy, it is necessary to fix 
upon some arbitrary unit such as the quantity of light produced 
by a lamp of definite form designated as the standard when 
operating under certain definite conditions. 

Obviously, the standard lamp must be of such a sort that 
the quantity of light emitted is not affected except by con¬ 
trollable conditions. 

Thus, the electric arc is useless for this purpose because 
even when furnished with electrical energy at fixed voltage 
and of fixed current value, the quantity of light varies enor¬ 
mously, due to the uncontrollable eccentricities of the arc. The 
most widely used standard is the Vernon-Harcourt Pentane 
lamp invented by the eminent English engineer of that name. 
When manufactured according to certain specifications con¬ 
cerning the sizes and arrangement of parts this lamp is used 
as the standard for measuring light. It is adjusted to burn 
a certain definite quantity of pentane per hour, and the baro¬ 
metric pressure and amount of moisture (humidity) and of 
carbon dioxide (COo) in the air are determined at the same 
time. 

Since by previous experiments the effects of different baro¬ 
metric pressures and amounts of moisture and of CO2 in the 
air are known, the results may be corrected to certain standard 
conditions. 

From the light given by this standard lamp the unit of light 
measurement must be derived. The total amount of light 
given out would seem to be the most rational value for this 
unit, but since the eye which is the only instrument sensitive 
to light, can only view the light source from one point, it is 
impossible to make a direct measurement or estimate of the 
total light emitted in this way. As the quantity of light given 
out is different in different directions, it is necessary to select 
one particular direction in which to measure the light. The 
horizontal has been selected for this purpose because it is 


UTILIZATION OF GAS APPLIANCE COURSE 


31 


easily determined and lends itself well to the construction and 
operation of photometrical apparatus. 

Having determined the kind of source and the direction 
which shall furnish the unit quantity of light, we must next 
determine the size of the beam to be used for this purpose. 
Just as a large pipe will under a certain pressure deliver more 
gas during a specified time than a small one, so a large beam 
of light from a certain source will contain more light than a 
small one. 

As a unit for measuring the size of a beam of light the 
solid angle is used. This is perhaps more easily illustrated 
than defined. 



The term angle signifies degree of divergence. For in¬ 
stance, if two straight lines meet at a point (Fig. i) they 
form an angle, and this angle is a measure of the divergence 
between the lines. The angle is measured by the ratio be¬ 
tween the arc (BC or B'C') of any circle drawn about the 
meeting point of the lines as a centre, and the radius of the 

BC B'C 

same circle AC or AC'. Thus, — or — is the angle expressed 

AC AC 

in radians. The term radian is used in scientific works be¬ 
cause of its greater simplicity for certain calculations. In 















32 


UTILIZATION OF GAS APPLIANCE COURSE 


practice angles are measured in degrees, one degree being 

I 

the angle measured by-th of the circumference of the 

360 

circle. For scientific purposes, however, the degree is un¬ 
wieldy and introduces complicated calculations. 

Thus, if in Fig. i a circle of 2' radius (4' diameter) be 
struck about A as a centre and the diverging lines AB and AC 
cut off an arc BC of i' in length on this circle, the angle is 
J = .5 radians. 

Similarly, if a circle of 3' radius be struck the arc B'C' v/ill 

1)4 

be 1)4' in length and the angle-= .5 radians. No matter 

3 

what size of circle be used, the expression for the angle between 
the two lines remains the same. 

The term “solid angle” is quite similar, except that instead 
of lines, we are dealing with planes, and instead of circles— 
spheres. Thus, if in Fig. 2 we have four lines AB, AC, AD, AE 
diverging from the point A, they will form the corners of a 
pyramid, the sides of which will cut off an area BCDE on the 
surface of any sphere described around the point A as a centre. 

Just as a plane angle is measured by the ratio of the arc to 
the radius of a circle, so the solid angle is measured by 
the ratio of the area cut off of any sphere to the square 
of the radius of the same, and this quantity will be constant 
for any given solid angle regardless of the diameter of the 
sphere. The unit for solid angle measurement is the steradian. 
If in Fig. 2 the radius of the sphere is 2 ft. and the area BCDE 

4 4 

contain four square feet the solid angle is— = - = i 

22 2x2 

steradian. 

No matter what form the area BCDE may take, whether 
it be cut off by a cone, a pyramid or an irregular shape bounded 
by lines diverging from “ A,” the solid angle at “ A ” is meas¬ 
ured in the same way. 




UTILIZATION OF GAS APPLIANCE COURSE 


33 


Applying this to the particular problem of a beam of light 
from a point source, it is easily seen that if the point A is emit¬ 
ting light equally in all directions inside the pyramid ABCDE, 
the smaller pyramid AFGHD will contain less than the larger 
pyramid, and in proportion to the areas FGHD and BCDF. 
That is, if FGHD is one-fourth of the area of BCDF, then 
the quantity of light in the smaller beam or pyramid will 
be one-fourth of that in the larger one. 

Having determined upon a standard light source, and a 
standard direction in which to measure it, it only remains 
to select a size of beam which we will fix upon as containing 
the quantity of light, which we shall use as a unit. We m.ay 
select for this purpose any solid angle we like, but it will be 
best to consider how the ease of mathematical calculations, 
etc., will be affected. 

In order to make the new unit conform in magnitude to the 
ones previously used, let us select a solid angle of i steradian* 

* In making actual measurements, however, it wiU be necessary to use 
a much smaller angle (say i-ioo steradian) since the distribution of light 
from the lamp varies considerably within a solid angle of i steradian. 
The measurement of quantity of light is based upon the fact that equal 
quantities of light falling upon surfaces of equal area and identical material 



will produce equal degrees of brightness—that is, upon the same surface, 
equal quantities of light per square inch will produce equal degrees of 
brightness. The apparatus used for measurements of this character is 
called a photometer and exists in a variety of forms. The Bunsen Photo- 














34 


UTILIZATION OF GAS APPLIANCE COURSE 


as representing the size of the standard beam of light, from 
the standard lamp in a horizontal direction. Let us take one- 
tenth the quantity of light in this beam in the unit for measur¬ 
ing and comparing quantities of light, since the present stand¬ 
ard lamp is lo times the power of the earlier standard. We 
may call it what we like, but must select a word not heretofore 
in use. 

The word lumen will serve to indicate the luminous nature 
of the commodity which we will measure in this way, and is 
distinctive and easily remembered and pronounced. Therefore 
we will call the new unit the lumen. 

This unit may be used to describe any quantity relating to 
light. For instance, if we wish to describe the brightness of a 


meter, the principle of which is illustrated above, is perhaps the form most 
widely used. It consists of a screen, the central portion of which is trans¬ 
lucent and the outer portion opaque. The screen was originally made by 
applying grease to the centre of a paper disc. Mirrors are provided 
enabling both sides of the screen to be seen at the same time. When 
equal quantities of light fall upon each side of the screen, the same degree 
of contrast between the centre and outer portion is shown upon each side. 
The apparatus is then said to be “in balance.” (Some authorities state 
that when “in balance” the central spot “disappears,” the whole field 
being of uniform brightness. That this is not and could not possibly be 
the case will be quite evident to any one who will take the trouble to think 
about the matter for a few minutes). 

When the screen is “ in balance ” the quantity of light per steradian from 
the lamp under test is calculated as follows: Suppose the lamp at A to emit 
light at the horizontal at one lumen per steradian. If the screen be 2" 

( 2 X 2 \ 

— - TZ I = . 0044 

30 X 30 / 


steradians and the quantity of light falling upon the side of the screen 
facing A is .0044 lumens. Since the same amount of light is also falling 
upon the side of the screen facing B, the quantity of light in the solid angle 

2X2' 


( 2 X 2 \ 

50 X 50 )^ ■ 


0016 


“b” is likewise .0044 lumens. The solid angle “b” is , 

^ \50x50) 

steradians. Since the light from B in .0016 steradians is .0044 lumens, 

. /.oo44\ 

the quantity light m one steradian will be I “^^^6) “ 2.8 lumens (nearly). 

It will be seen that in these calculations the area of the screen and the 
illumination is the same in both cases. These quantities therefore dis¬ 
appear and the final calculations may be made by using only the squares 


of the respective distances from the lamps to the screen. ThusI 
(nearly). 








UTILIZATION OF GAS APPLIANCE COURSE 


35 


flame, a mantle or an illuminated wall or ceiling, we may do 
SO in terms of lumens per steradian per square inch or per 
square foot, meaning the quantity of light emitted per unit 
area in a given direction. Obviously, i lumen emitted by an 
area of one square inch will produce a brighter surface than 
I lumen emitted by lo square feet. On the other hand, if 
we wish to describe the degree to which illumination is con¬ 
centrated upon a lighted area, we may again express it in 
lumens per square foot, meaning the quantity of light received 
by each square foot of the surface. The difference between 
the amount of light received by a surface (not self-luminous) 
and that emitted by it is the amount absorbed by the surface. 

Excepting color, the only characteristics of illuminated sur¬ 
faces in which we are interested is the distribution of surface 
brightness, and it is plainly more sensible to express this by the 
use of a unit which also designates the quantity of light avail¬ 
able for illumination, than to use units (candle-power, foot- 
candles, etc.) which must be transformed into more logical 
units before they can be used to convey any intelligible idea 
concerning illumination. In order to avoid confusion, how¬ 
ever, the value of the lumen has been so selected that one 
lumen per square foot is equal to one foot-candle.* 

If artificial light sources gave out equal quantities of light 
in all directions, a simple statement of the quantity in any one 
direction would convey sufficient information concerning the 
characteristics of a lamp. This is not the case, however, so 
it is necessary to devise some means of showing the quantities 
given out in various directions in a simple and convenient form. 
Such information may be conveyed in the form of tables in 
which the quantity of light in a beam of standard size in each 
direction is shown. If we designate the directions by angles 
we must first select a certain direction to be designated by 
zero as a point of reference. The direction of a point im¬ 
mediately below the lamp is universally used as the zero or 

* The use of the term candle-power is perfectly logical if it is regarded 
only as a short equivalent for i lumen per steradian and not as a funda¬ 
mental or basic unit of light measurement. 



36 


UTILIZATION OF GAS APPLIANCE COURSE 


reference point and the horizontal is designated by 90°. Sup¬ 
pose that in a beam of i/ioo steradian (a beam so small that 
we may expect the distribution of light to be uniform within 
it) directly downward from the lamp shown in Fig. 3 the 
quantity of light is found to be .88 lumens; at the horizontal, 



.34 lumens; midway between (at 45°), 136 lumens; directly 
upward (180°), .15 lumens, and at 135°, 35 lumens. 

We may tabulate this information as below: 


Angle 

I 

Quantity of Light in 100 Steradian 

Quantity of Light 

per Steradian 

0°. 

.88 lumens 

88 lumens 

45 °. 

1.36 “ 

136 “ 

90°.... 

■34 

34 - :: 

135. 

•35 “ 

35 

i8o‘’. 

•15 

15 


For most purposes, however, it is necessary to know the 
quantities of light at many intermediate angles. 

Such a table would occupy much space and would not con¬ 
vey a very graphic idea of the manner in which the light is 
distributed. 


















UTILIZATION OF GAS APPLIANCE COURSE 


37 


o 

<0 



We therefore substitute for the column of angles a chart 
composed of lines actually drawn at the angles required, and 
place upon each line a figure representing the quantity of light 
in that direction (Fig. 4). 

But this arrangement, though economizing space, does not 
■show at a glance distribution of light. It is still necessary to 
pick out the different figures and compare them, so we adopt 
the expedient of placing each figure at a distance from the cen¬ 
tre proportional to the amount designated (Fig. 5). This, 
however, is a rather awkward form, as in order to show the 
values at all intermediate angles, the number of lines and figures 
becomes confusingly large. We therefore drop the use of fig¬ 
ures entirely, placing dots at the distances from the centre 



Fie. 5 






38 


UTILIZATION OF GAS APPLIANCE COURSE 


proportional to the quantities of light and connect all these 
dots by a smooth curve. We place a scale (Fig. 6) by which 
these values may be determined upon the vertical line and 
provide circular lines by means of which this scale may be 
transferred upon any line representing an angle, and the 
quantity of light read at a glance, the form of the curve show¬ 
ing the characteristics of light distribution for this particular 
lamp and reflector. 

It will be seen that as we have determined the quantities of 
light at diflerent angles in one vertical plane only (Fig. 7), we 
have no means as yet of finding the total amount of light given 
by the lamp. This will receive attention later. 

Having determined the quantities of light in beams of stand¬ 
ard size at different angles, and arranged the results in a con¬ 
venient form for reference, we are next interested in ascer¬ 
taining the degree to which this light will be concentrated over 



Fig. 6. 
















UTILIZATION OF GAS APPLIANCE COURSE 


39 



surfaces in different positions and at different distances. If 
our light source is so small that it practically may be regarded 
as a point emitting light in all directions, the area covered by 
each beam will be greater as the distance from the lamp in¬ 
creases (Fig. 8) and also as the angle of inclination between 
the beam and the illuminated surface increases (Fig. 9). If 
the surfaces are perpendicular to the beam, it is quite easy to 
show that at 2 feet from the lamp a beam of a given size will 
cover four times as great an area as at i foot (Fig. 8), and 
at 3 feet, nine times as great. In other words, the area cov¬ 
ered is proportional to the square of the distance, and as the 
greater the area covered by a given quantity of light, the more 



















40 


UTILIZATION OF GAS APPLIANCE COURSE 


thinly it will be spread (so to speak) and the less brightly the 
surface will be illuminated. The illumination is said to vary 
inversely as the square of the distance, and by some writers 
this is called the “Law of Inverse Squares.” It is nothing 
of the sort. It is merely a statement of mathematical rela¬ 
tions having nothing whatever to do with the laws of light. 

It may properly be called the Rule of Inverse Squares, bear¬ 
ing in mind that it refers only to single light sources, which are 
practically points emitting light in diverging rays. 

The extent to which the areas covered by a beam of light 
are increased by inclining the lighted surface to the beam is 
not susceptible of such simple statement, and for practical pur¬ 
poses it is more convenient to make use of the tables of con¬ 



stants or factors supplied for such calculations in the various 
hand and text-books available. 

These tables are usually gotten out in the form of factors 
by which to multiply the light quantities at angles designated 
in the tables as corresponding to certain distances below and out 
from the lamp, and give results in terms of lumens per square 
foot. 

Thus (see Table i) if we wish to determine the illumination 
on a horizontal plane at a point 8 feet out from directly be¬ 
neath the lamp (the distribution curve of which is shown in 
Fig. 6) and in a plane 5 feet below the lamp, we find from 
the table that the factor is .00595 and the angle 58°. (See 
Table i.) 



















UTILIZATION OF GAS APPLIANCE COURSE 


41 


The figure .00595 is the reciprocal * of the area covered by 
the beam at this distance and angle of inclination. 

This is used because multiplication is easier than di- 
division. The area covered is approximately 168.067 sq. ft. 
Since multiplying by .00595 produces the same result as 
dividing by 168.067 and the operation is more quickly and 
easily performed, these factors are given in the form of re¬ 
ciprocals by which the quantity of light at the angle specified 
must be multiplied. Thus, we find the illumination on a hori¬ 
zontal plane at the point designated to be 360 X *00595 = 
.342 lumens per sq. ft. To find the normal illumination, or 
that at the same place upon a plane squarely facing the light 
source, use the proper factor, .0112 (Table i). 360 X 
.0112 = 4.032 lumens per sq. ft.—the normal illumination. 
Notice that this calculation holds only for a single point, or 
rather a small area at the designated location, and not over 
the entire surface covered by the beam of standard size (i 
steradian. This size was adopted purely for convenience in or¬ 
der to make the new units conform tO' the old in magnitude. 
The actual measurements of light quantity are made upon a 
much smaller beam (i/ioo steradian or less). This in nowise 
affects the accuracy of the results, merely the magnitude of the 
mathematical quantities under consideration, which are so 
selected as to eliminate discrepancies in final results. 

As stated above, the new units have been so selected that 
they may be used in connection with old candle-power curves 
and foot-candle data. Thus: 

I lumen per steradian = i candle-power. 

I lumen per sq. ft. = i foot-candle. 

In this way no confusion will arise in using the new and 
more logical units in place of the old. 

*The reciprocal of a number is i divided by the number. Multiplying 
by the reciprocal of a number is the same as dividing by the number. 
Thus, the reciprocal of 4 is i = .25. Multiplying by .25 is the same as 
dividing by 4. 





TABLE I. 

TABLE OF ANGLES, HORIZONTAL AND NORMAL FOOT-CANDLE CONSTANTS FOR DIFFERENT HEIGHTS AND DISTANCES, 


Cf 




e>9 


eq 


85 ° 15 ' 

.000142 

.00171 

82 ° 50 ' 

.000215 

.00172 

80 ° 30 ' 

.000281 

.00170 

78 ° 15 ' 

.000338 

.00166 

76 ° 0 ' 
.000393 
.00162 

73 ° 45 ' 

.000447 

.00159 

71 ° 35 ' 

.000493 

.00156 

84 ° 50 ' 

.000183 

.00203 

82 ° 15 ' 

.000272 

.00202 

79 ° 40 ' 

.000361 

.00201 

77 ° 10 ' 

.000438 

.00197 

74 ° 45 ' 

.000505 

.00192 

72 ° 20 ' 

.000570 

.00188 

70 ° 0 ' 

.000625 

.00183 

84 ° 20 ' 

.000242 

.00245 

81 ° 30 ' 

.000359 

.00243 

78 ° 40 ' 

.000474 

.00241 

76 ° 0 ' 

.000566 

.00234 

73 ° 20 ' 

.000655 

.00228 

70 ° 45 ' 

.000731 

.00222 

68 ° 10 ' 

.000805 

.00216 

83 ° 40 ' 

.000341 

.00309 

80 ° 30 ' 

.000500 

.00303 

77 ° 30 ' 

.000631 

.00291 

74 ° 30 ' 

.000763 

,00285 

71 ° 35 ' 

.000876 

.00277 

68 ° 45 ' 

.000972 

.00268 

66 ° 0 ' 
.00105 
.00258 

82 ° 55 ' 

.000469 

.00380 

79 ° 55 ' 

.000688 

.00374 

76 ° 0 ' 

.000885 

.00366 

72 ° 40 ' 

.00106 

.00356 

69 ° 25 ' 

.00121 

.00344 

66 ° 20 ' 

.00132 

.00329 

63 ° 25 ' 

.00140 

.00313 

81 ° 50 ' 

.000717 

.00505 

77 ° 55 ' 

.00102 

.00487 

74 ° 5 ' 
.00129 
.00470 

70 ° 20 ' 

.00152 

.00452 

66 ° 50 ' 

.00169 

.00429 

63 ° 25 ' 

.00183 

.00409 

60 ° 15 ' 

.00191 

.00385 

80 ° 30 ' 

.00112 

.00678 

Is. Oi 

O lO 

1-H CO 
?n O O 

J 2 o o 

71 ° 35 ' 

.00197 

.00624 

67 ° 25 ' 

.00226 

.00588 

63 ° 25 ' 

.00249 

.00556 

59 ° 45 ' 

.00261 

.00518 

56 ° 20 ' 

.00266 

.00480 

78 ° 40 ' 

.00191 

.00972 

73 ° 20 ' 

.00262 

.00913 

68 ° 10 ' 

.00321 

.00863 

63 ° 25 ' 

.00358 

.00800 

59 ° 0 ' 
.00379 
.00736 

55 ° 0 ' 
.00385 
.00671 

51 ° 20 ' 

.00381 

.00610 

77 ° 30 ' 

.00253 

.0117 

71 ° 35 ' 

.00350 

.0111 

66 ° 0 ' 
.00421 
.0103 

60 ° 55 ' 

.00459 

,0944 

56 ° 20 ' 

.00473 

.00853 

52 ° 10 ' 

.00471 

.00767 

48 ° 25 ' 

.00459 

.00691 

76 ° 0 ' 
.00354 
.0146 

69 ° 25 ' 

.00483 

.0137 

63 ° 25 ' 

.00560 

.0125 

58 ° 0 ' 
.00595 
.0112 

53 ° 10 ' 

.00598 

.00997 

48 ° 50 ' 

.00582 

.00884 

45 ° 0 ' 
.00552 
.00781 

74 ° 5 ' 
.00516 
.0188 

66 ° 50 ' 

.00677 

.0172 

60 ° 15 ' 

.00764 

.0154 

54 ° 30 ' 

.00783 

,0135 

49 ° 25 ' 

.00765 

.0118 

45 ° 0 ' 
.00722 
.0102 

41 ° 10 ' 

.00667 

.00886 

71 ° 35 ' 

.00788 

.0249 

63 ° 25 ' 

.00996 

.0222 

66 ° 20 ' 

.0106 

.0191 

50 ° 10 ' 

.0105 

.0164 

45 ° 0 ' 
.00982 
.0139 

40 ° 35 ' 

.00894 

.0118 

36 ° 50 ' 

.00801 

.0100 

68 ° 10 ' 

.0129 

.0347 

59 ° 0 ' 

.0152 

.0295 

51 ° 20 ' 

.0152 

.0239 

45 ° 0 ' 

.0141 

.0199 

39 ° 50 ' 

.0126 

.0164 

35 ° 30 ' 

.0110 

.0135 

32 ° 0 ' 
.00953 
.0112 

63 ° 25 ' 

.0224 

.0500 

53 ° 10 ' 

.0241 

.0402 

45 ° 0 ' 

.0221 

.0312 

38 ° 40 ' 

.0190 

.0243 

33 ° 40 ' 

.0160 

.0192 

29 ° 45 ' 

.0134 

.0154 

26 ° 35 ' 

.0112 

.0125 

56 ° 20 ' 

.0426 

.0768 

45 ° 0 ' 

.0393 

.0555 

36 ° 50 ' 

.0320 

.0399 

31 ° 0 ' 

.0252 

.0294 

26 ° 35 ' 

.0199 

.0222 

23 ° 10 ' 

.0159 

.0173 

20 ° 35 ' 

.0128 

.0136 

45 ° 0 ' 

.0884 

.125 

33 ° 40 ' 

.0641 

.0770 

26 ° 35 ' 

.0447 

.0500 

21 ° 50 ' 

.0320 

.0344 

18 ° 25 ' 

.0237 

.0250 

15 ° 55 ' 

.0181 

.0188 

14 ° 0 ' 

.0143 

.0147 

26 ° 35 ' 

.179 

.200 

18 ° 25 ' 

.0949 

.100 

14 ° 0 ' 

.0571 

.0588 

11 ° 20 ' 

.0377 

.0384 

9 ° 30 ' 

.0266 

.0269 

8 ° 10 ' 

.0198 

.0200 

7 ° 10 ' 

.0153 

.0154 

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® (N M 

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.0625 

0 ° 

.0400 

.0400 

0 ° 

.0278 

.0278 

0 ° 

.0204 

.0204 

o 

.0156 

.0156 

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0 ° 6 ° 20 ' 12 ° 30 ' 18 ° 25 ' 24 ° 0 ' 29 ° 5 ' 33 ° 40 ' 37 ° 50 ' 41 ° 40 ' 45 ° 0 ' 48 ° 0 ' 53 ° 10 ' 67 ° 15 ' 60 ° 40 ' 63 ° 25 ' 65 ° 45 ' 67 ° 45 ' 69 ° 25 ' 

.0123 .0121 .0115 .0105 .00941 .00824 .00712 .00608 .00515 .00436 .00370 .00266 .00195 .00145 .00111 .000855 .000670 .000536 

.0123 .0122 .0118 .0110 .0103 .00943 .00855 .00770 .00689 .00616 .00556 .00444 .00360 .00296 .00248 .00208 .00177 .00152 
































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-S^ S 3 g a 


^ d ^ 


o 


^ 05 d d**^ 

o ^ C 3 C 3 ri 
^ ft C5 C5 9 

if 05 O 

g 05 'd 

(ij ft,d,d*d 
_S ft C 5 C 5 s 

o 05 ^ ^ d: 

-SiS 2 
o 

*5 ,►» 

























44 


UTILIZATION OF GAS APPLIANCE COURSE 


In determining the total amount of light given out by a 
lamp it is necessary to consider the fact that at any angle 
near the horizontal there are many more beams of a given 
size than at angles nearer the vertical. This is plainly shown 
in Fig. lo. That is, each angle represents a belt or zone, and 



the area of this belt or zone increases rapidly as the angle 
increases up to 90°. Beyond 90° the reverse takes place. 

By mathematical means which need not be explained here 
the total quantity of light may be calculated from any sym¬ 
metrical distribution curve, that is, from the curve of a lamp 
which distributes light in the same way in all vertical planes. 
















UTILIZATION OF GAS APPLIANCE COURSE 


45 


Fig. iia shows typical non-symmetrical distribution. 

Information concerning the total quantity of light, and that 
in the upper and lower hemispheres, is usually furnished with 
manufacturers’ distribution curves, and there is little necessity 
for the salesman to make calculations of this sort. 



Fig. iia. 

Full and dotted lines show distributions in two vertical planes at right 
angles to each other. 

Without explaining the mathematical basis of these calcu¬ 
lations, it may be well to give the necessary rules for calcu¬ 
lating the quantity of light in any zone from any lamp, which 
is as follows: 









































46 


UTILIZATION OF GAS APPLIANCE COURSE 


To obtain the amount of light in any zone io° wide (ex¬ 
tending 5° on either side of any angle), multiply the quantity 
of light in a beam of standard size at that angle by the factor 
in the table below: 


Angle 

Zone Extending 
From To 

Factor 

n® nr 

0° 

* 5 ° 

.0240 


175° 

i8o°* 


5 ° 

15° 

. 1900 


165° 

175° 

on® nr ifin® 

15° 

25° 

.3860 


155° 

165° 

10® nr T cn® 

25° 

35 ° 

•5940 

^ 0^ * . 

145° 

155° 

/^n® nr T/jn® 

35 ° 

45 ° 

.7190 


135° 

145° 

cn® nr t on® 

45 ° 

55 ° 

.8600 

O'-' ^ 0^ * . 

125° 

135° 

fin® nr Ton® 

55 ° 

65° 

•9550 


115° 

125° 

yn® nr t rn® 

65° 

75 ° 

I.0300 


105° 

115° 

Sn® nr inn® 

75 ° 

85° 

I.0700 


95 ° 

105° 

90°. 

85° 

os'" 

I.0900 


yo 


After calculating the quantity of light in each zone, add 
together to obtain the total up to any desired angle. 


*Since the zone extending 5° to either side of 0° is embraced by a zone 
5° wide, the latter must be used at this point. 






































UTILIZATION OF GAS AFPLIANCE COURSE 


47 


For instance, to obtain the total light up to 90°, add the 
results for the following angles 0°, 10°, 20°, 30°, 40°, 50°, 60°, 
70°, 80° and one-half that for 90° (since the 90° calculation 
includes all light from 85° to 95°). 

In order to eliminate the necessity of making these calcula¬ 
tions the curves shown in Fig. ii have been prepared. On the 
right hand is shown the ordinary distribution curve from an 
opaline reflector of the extensive type and also a dotted curve 
showing the ideal extensive distribution to obtain uniform 
illumination on both floor and walls with lamps spaced a 
distance apart equal to twice their height above the floor. On 
the left-hand side is shown a curve (A) in which the reading 
of the curve at any angle, according to the scale shown, gives 
the total quantity of light in a zone 10° wide, 5° to each side 
of the designated angle. On the left-hand side is also a 
curve (B) which at any angle shows the total quantity of 
light from 0° up to and including the designated angle. Thus 
at 90° is shown the total light below the horizontal and at 
180° the total light from the lamp. Comparing the left-hand 
curve (A) with the ordinary distribution curve of the same 
lamp shown on the right a great difference will be noted. 
This is due to the fact that the left-hand curve is what may 
be called a plane distribution curve, showing the quantity of 
light in one vertical plane only. It does not show all the 
light emitted, but only that in the direction of a vertical band 
(Fig. 7) encircling the lamp, while the right-hand curve shows 
all the light given by the lamp at any angle. The latter, there¬ 
fore, gives a much better idea of the manner in which the 
light is distributed than the former. It is seen that even 
though at any one horizontal direction the quantity of light 
appears from Fig. ii—left hand—to be quite small, that really 
a very large proportion of the total light is given out in horizon¬ 
tal directions. 

Note particularly that while, according to curve C, most of 
the light appears to be distributed below 60° from the vertical 
(usually regarded as the ‘"useful zone”). In reality nearly 
one-half of the light is given out above this angle. 


48 


UTILIZATION OF GAS APPLIANCE COURSE 



After the rays of light leave the lamp they will travel in 
straight lines and the quantity of light will not be diminished 
unless some substance exerting a modifying influence is en- 
-countered. 

1 The quantity)’ direction and color of light may be modified 
in the following ways: ’ 

Absorption, by which the quantify of light is diminished 
































UTILIZATION OF GAS APPLIANCE COURSE 


49 



a£>^orp/Jor} and fransm i ssion 
r I a 12 

(partial absorption, Fig. 12) or totally absorbed (Fig. 13). Ab- 
sorption may or may not affect the color of the light. Where 
absorption is only partial, there is also some transmission or 
reflection. 



BiacK Absorption. 

FiQ. 13 
















50 


UTILIZATION OF GAS APPLIANCE COURSE 



TofaJ TransmJtJon reguJ^r reflectior 

Tig. 14 FigJS 

Transmission (Fig. 14)—the reverse of absorption. 

Reflection, by which the direction is altered, the light not 
penetrating beneath the surface of the substance. 

Reflection may be either regular (Fig. 15) or diffuse (Fig. 
16). Regularly reflected light rays always leave the surface 
at the same angle at which they strike it. Diffuse reflection 
is simply a number of regular reflections from small sur¬ 
faces at different angles or not in the same plane. 

Refraction (Fig. 17), by which the direction of the ray is 
altered by passing from a transparent medium of one density 
into one of another density (air to glass, glass to air, etc.). 
The angle of refraction depends upon the relative densities of 
the substances. 

If in attempting to pass from a denser into a lighter sub¬ 
stance, the light rays strike the surface at such an angle that 
refraction directs the light back into the denser medium, no 



kfhite Planter 


Parf/a/e/Z/fuse re/Jec/Jon £ absorption. 
Fis. 16 . 



TotaJ mfefno/ re//ec/idr> 

la 















UTILIZATION OF GAS APPLIANCE COURSE 



Re//"a c//on LUJf/7 por//aJ regu/or 
refjectjon 
Fig. 17. 




/?efracf/or7 coU /7 parf/a/ rega/ar 
rof/ecf/oo & rota/ regu/ar ref/ec//on. 









52 


UTILIZATION OF. GAS APPLIANCE COURSE 



Fig. i8a. 





UTILIZATION OF a^S APPLIANCE COURSE 


53 



light escapes and total internal reflection is said to take place 
(Fig. i8). The “critical angle” at which this occurs is illus¬ 
trated by OEG (Fig. i8a). 

. The action of prismatic glass reflectors depends upon 
this principle (Fig. 19). 

As stated before, the eye perceives objects and their details 
largely through such contrasts of brightness as result from the 
form, texture, etc., of the object. 

While the subject of reflection may be regarded as of rather 
academic interest, it is worth while to note the application of a 
few principles to reflector design. 

Figs. 20, 21 and 22 show the type of reflection from polished 
aluminum, frosted aluminum, and white enameled steel, re¬ 
spectively. 




riG. 20 


Fia 21 










54 


UTILIZATION OF CAS APPLIANCE COURSE 



ri6.22 


r/6.23 


It is evident from Fig. 20 that all the light in any ray may 
be redirected in any desired direction by arranging the reflect¬ 
ing surface in the proper position with relation to it. Thus, 
if desired to direct all the light in one direction in a very 
narrow beam (as in a search-light), the reflector may be formed 
as in Fig. 23. Therefore, reflectors of this character are al¬ 
ways made of polished metal or silvered glass. 

This result may be accomplished to a less extent by the use 
of frosted aluminum, because of the considerable proportion 
of regular reflection present (Fig. 21). It is quite apparent, 
however, that very little control over the direction of the re¬ 
directed rays may be secured by the use of enameled surfaces. 
No matter what their shape, all that they can accomplish is to 
reflect back a portion of the light in the general direction of the 
open side of the reflector (Fig. 24). They are therefore use- 



Djffusely reflecting surface. 
riG.Z 4 . 


































UTILIZATION OF GAS APPLIANCE COURSE 


55 


less for any purpose demanding any considerable degree of 
light concentration. 

Most of the glass reflectors used for commercial lighting 
combine both regular and diffuse reflection which diffuse trans¬ 
mission. They thus afford some (and in many cases consider¬ 
able) control over the distribution of light. 



Fig. 26. 


Those which act as efficient reflectors, however, are usually 
lacking in transmission, and do not provide sufficient light at 
and above the horizontal (Fig. 26) to make their indiscrimi¬ 
nate use advisable with gas lamps. In the case of electric 
lamps placed a very short distance below the ceiling, this 
deficiency is less objectionable. 






















































UTILIZATION OF GAS APPLIANCE COURSE 


S6 


Ball globes (Fig. 28), on the other hand, furnish many more 
times the wall and ceiling illumination than is ordinarily neces¬ 
sary, or even desirable, and do not distribute the light ad¬ 
vantageously in the lower part of the room, where it is usually 
most needed. 

The amount of light absorbed by reflecting surfaces (either 
the equipment of the lamps themselves or the walls and ceil¬ 
ings of the room) depends upon the nature and color of the 
surfaces. It is impossible to make any estimate of this quality 
from the appearances of the surfaces. 



Fig. 27. 



















































UTILIZATION OF GAS APPLIANCE COURSE 


57 


The following table gives the proportion of light falling upon 
them absorbed by various surfaces: 


Material 

Percentage of Light 
Absorbed 

Polished silver. . . 

7 to 8% 

12 to l8% 
i8% 

20 % 

25 to 30 % 

30 % 

6 o% 

75% 

87% 

88% 

99 - 6 % 

Silvered mirror. 

White blotting paper. 

White cartridge paper. 

Polished brass. 

Mirror backed with amalgam. 

Yellow wall paper. 

Light blue cardboard. 

Dark brown paper. 

Vermilion paper. 

Black velvet. 



Note the small difference in absorption between white paper 
and a silvered mirror, and between dark brown and vermilion 
paper—quite the opposite result from what one would antici¬ 
pate. Observe also the enormous difference between white 
and light blue cardboard. It is quite evident that estimates of 
the effects of light and dark walls upon illumination are very 
much a matter of guesswork, unless the color and shade be 
rather precisely stated. 

The most important calculations in illumination deal with 
the light after it has left the lamp. 

The salesman must choose from a comparatively small num¬ 
ber of lamps and shades in planning lighting installations, and 
these have fixed characteristics which he is not able to control. 
He is less interested in how the light is produced and reflected 
or diffused from the globe or shade than in the effect it will 
produce upon objects on which it falls. 

For most purposes the excellence of illumination is regarded 
as depending largely (though not entirely) upon the intensity 
of illumination on the “ working plane,” which is the plane 
containing the objects requiring illumination—the tops of desks 
or tables in office-lighting, the walls in an art-store, etc. There 
are really six “ working planes ” in most cases—the floor (or a 



















UTILIZATION OF GAS APPLIANCE COURSE 


S8 



Fig. 28. 

certain distance above it), ceiling and four walls. Since sep¬ 
arate calculations in these six planes would entail an amount 
of labor out of all proportion to their value, and in most cases 
a horizontal plane (usually about 30 inches above the floor) 
is the most important, manufacturers have designed their com¬ 
mercial reflectors for uniform and efflcient illumination of a 
horizontal plane. At the same time, sufficient light is pro¬ 
vided near and above the horizontal to furnish adequate il¬ 
lumination of walls and ceiling for all purposes except where 
these form the most important planes for illumination. These 































UTILIZATION OF GAS APPLIANCE COURSE 


59 



Fig. 29. 


exceptions are very few and the salesman, unless he be a spe¬ 
cialist, is most unlikely to encounter them. 

Provided anything approaching ordinary good judgment is 
used in the choice of reflectors the salesman will accomplish 
the best results by basing calculations upon the illumination 
of the horizontal plane. For this purpose, he may employ 
either or both of two methods. The first, or “ Point-by-Point 
Method,” involves the calculation of illumination at various 
points in the “ working plane ” by the method described on 
page 41. The results of these calculations are often shown 
in the form of illumination curves (Fig. 25). In these curves. 






















































Lower Curve—Illumination from old system. 




















































































UTILIZATION OF GAS APPLIANCE COURSE 


6i 


the base line (A-A) represents a line in the working plane, 
and the height of the curve above any point in the base line 
represents, to the scale at the end, the illumination at that 
point. 

Such curves take into consideration the direct light from the 
lamps only, and disregard that reflected from side-walls and 
ceiling. Similar curves are also used to show the results of 
actual measurements of illumination, in which case the light 
from ceiling and walls is of course included. 

That this method is extremely laborious is self-evident. 

The second, or “ Flux Method,” is based upon the results of 
actual tests in rooms of various shapes and sizes, which have 
shown that with a given type of lamp and reflector and walls 
and ceiling of given color, the percentage (or proportion) of 
the total light produced that will reach the horizontal working 
plane is practically the same in any shape or size of room 
(within reasonable limits, of course). 

From a sufficient number of such tests it is easy to compile a 
table showing the percentage of total light produced which 
reaches the working plane with various types of reflectors, 
with walls at various colors. 

Such a table follows: 


Reflector or Globe 

Percentage of Total 
Light from Bare lamp 
Reaching Working 
Plane 

White Ceiling 

Per Cent. 
Lost by 
Black 
Walls 

White 

Walls 

Black 

Walls 

Clear prismatic reflector (Fig. 26 ) (E). 

65 

55 

15 

Light alabaster reflector (Fig. 27 ).. . 

55 

45 

20 

Light alabaster ball globe (Fig. 28 ). . 

50 

35 

30 

Semi-indirect (Fig. 29 ). 

40 

30 

25 

Indirect (Fig. 30 ). 

35 

25 

30 


















62 


UTILIZATION OF GAS APPLIANCE COURSE 



Fig. 30. 


These figures are typical only. Globes and reflectors of 
somewhat different design or quality would give somewhat 
different results. 

Electric lamps will give results differing from gas lamps, 
since the original distribution of light from the lamp is some¬ 
what different. 

For practical purposes, however, they are sufficiently accurate. 

As would be expected, dark walls have the least effect where 
clear prismatic reflectors are used, since a comparatively small 
proportion of the light is given out near the horizontal. The 























UTILIZATION OF GAS APPLIANCE COURSE 


63 


ball globe naturally suffers most. The semi-indirect system 
suffers less than the indirect, since a considerable proportion 
of the light is sent directly to the working plane. 

To use this Table it is necessary first to calculate the total 
amount of light (in lumens) given by the bare lamps. Multi¬ 
plying this by the percentage of total light reaching the hori¬ 
zontal working plane gives the total light upon the plane, and 
dividing by the area of the plane (usually that of the floor) 
gives the lumens per square foot or the average intensity of 
illumination. 

A variation of this method is the tabulation of the total 
light reaching the working plane from different lamps, instead 
of percentages. This eliminates the first of the above calcu¬ 
lations. 

Such a table is shown below: 


Single Mantle Inverted Unit 
( 4 cu. ft. per hour size) 

With 

Amount of Light per Lamp 
Reaching Working Plane 

White Ceilings 

White Walls 

Black Walls 

Clear prismatic reflector. 

520 

440 

Alba reflector.. 

440 

360 

Light alabaster ball globe. 

400 

280 

Semi-indirect. 

320 

240 


These figures differ somewhat from previously published data on 
account of changes in mantle and reflector design, which have resulted 
in changes in light distribution. 

Another variation of this method is the tabulation of the 
amount of light reaching the working plane per cubic foot 
of gas consumed per hour. 

It is not necessary to illustrate this Table, as it is compiled 
by dividing the figures in the above table by the hourly con¬ 
sumption of the lamp. 

















64 


UTILIZATION OF GAS APPLIANCE COURSE 


Many other modifications of this method may be made to 
simplify calculations in the design of lighting systems, all of 
which are based upon the same principles. 

This method gives no information as to the uniformity of 
illumination. A room may be lighted to a very high average 
of illumination and yet be very poorly lighted in certain por¬ 
tions. Fig. 31 shows the illumination obtained from two dif¬ 
ferent lamp equipments, giving the same average illumination. 

At the point midway between the lamps the illumination from 



/ 




















UTILIZATION OF GAS APPLIANCE COURSE 


6S 



Fig. 32. 

one is far below that from the other, though much greater 
below the lamp. Curve represents over one-third more 
effective illumination than curve Ij. 

Wherever the same sort of work is to be done in all parts 
of a room, sufficient illumination for the purpose must be 
provided in all portions of the lighted area, and nothing is 
gained by illuminating any portion to a higher degree than is 
adequate for the purpose. In the interests of economy, there¬ 
fore, uniformity of illumination is preferable. In order to as¬ 
sist the salesman in securing this result, reflector manufac- 























66 


UTILIZATION OF GAS APPLIANCE COURSE 


turers design their product so that certain combinations of 
reflectors, outlet spacing and lamp, heights (above the work¬ 
ing plane) will give the highest uniformity of illumination ob¬ 
tainable. 

Fig. 26 shows three types of distribution. Extensive (E) 
Reflectors will give the most uniform illumination when hung 
at a distance above the working plane equal to one-half the 
distance between outlets. Intensive (I) reflectors should be 
placed at a distance above the plane equal to four-fifths their 
distance apart, and Focusing (F) Reflectors one and one-third 
times the distance apart. 

Fig. 32 shows a type of distribution having what is 
loosely termed Distributing Characteristics. It would seem 
from the name and general appearance that lamps fitted with 
such equipment might be placed further apart than with the 
Extensive type. 

A little consideration of the principles of illumination laid 
down on page 40 will show that the reverse is the case. 



Fig. 33. 









































UTILIZATION OF GAS APPLIANCE COURSE 


67 


As the distance of a horizontal surface from a point directly 
beneath the lamp increases, its angle of inclination to the light 
rays, and hence the amount of light required to illuminate it, 
increase with greater and greater rapidity. Thus, at a point 
sixteen feet distant from beneath a lamp five feet above the 
plane, over four times as much light is required as for the 
same illumination at eight feet distance. 

For a reflector to permit wider spacing than with the Ex¬ 
tensive type, therefore it would require a distribution some¬ 
what as shown in Fig. 33. 

As a matter of fact, Distributing shades and reflectors must 
be placed closer together than the Extensive type—generally 
speaking, about one and two-thirds times the height above 
the plane. That is, the height above the plane should be two- 
thirds the distance apart. 

Generally speaking, with a given type of shade, the closer 
together the lamps are placed, the more uniform the illumina¬ 
tion and the greater the freedom from shadows, especially 
where walls and ceilings are dark. Light walls and ceilings 
contribute by diffuse reflection a uniform illumination that 
tends to reduce the non-uniformity of that received directly 
from the lamps. 

In many cases, however, uniformity of illumination is not 
the most important consideration, nor is the horizontal plane 
always the most important one. The character of distribu¬ 
tion required will depend upon the class of service and the 
conditions to be met. 


68 


UTILIZATION OF GAS APPLIANCE COURSE 


PART 111 

TABLES 

The following tables are typical, but not sufficiently com¬ 
plete for use in the design of appliances. There is also a con¬ 
siderable lack of agreement among the results obtained by dif¬ 
ferent authorities. 

Both Specific Heat and Thermal Conductivity vary with the 
temperature of the substance and its condition, sometimes very 
widely. 

The examples given are intended merely to illustrate the 
procedure. The calculations required in actual practice are usu¬ 
ally much more complicated. 


SPECIFIC HEAT 

Air (at constant volume).16847 

Carbon dioxide (at constant volume).1535 

Oxygen (at constant volume).I 5507 

Nitrogen (at constant volume).17273 

Hydrogen (at constant volume). 2.41226 

Water . i.00000 

Mercury.0333 

Cast iron.1298 

Copper.0951 

Ice.5040 


To obtain the amount of heat required to raise the temperature of 
a given quantity of any of the above substances by any amount, mul¬ 
tiply the weight in pounds by the increase in temperature and by the 
figure from the Table. 

Example: How much heat will be required to raise i lb. of mercury 
from 32° F. to 50° F. ? 

(50 — 32 = 18) X.0333 = .2997 B. T. U. 












UTILIZATION OF GAS APPLIANCE COURSE 


69 


THERMAL CONDUCTIVITY AT 0° F. 


Air . 1.65 

Brick . 6.00 

Asbestos . 1.00 

Copper . 555-00 

Cotton wool. 1.25 

Ground chalk . 5.80 

Iron . 232.00 


To obtain the number of B. T. U.’s per hour transmitted through any 
of the above substances, multiply the area or cross section in sq. ft. 
of the transmitting material by the difference in temperature in degrees 
Fahr. between the two sides or ends and by the constant in the above 
table, and divide the result by the thickness or length in inches. 

Example: How much heat per hour will be transmitted through an 
iron bar 1" X 2" (or 1/12' X 1/6') in cross section, and 8" long, 
having a temperature of 390° F. at one end and 320° F.* at the other? 

1/12 X 1/6 X (390 — 320 = 70) X 232 -^8 = 28.4 B. T. U. per hour. 

Example: How much heat per hour will be transmitted through a 
sheet of asbestos 2' square (2X2 = 4' sq. ft.) and h" thick, having 
a temperature of 350° on one side and 300° on the other? 

4 ^ (350 — 300 = 50) X I. 8 = 1600 B. T. U.’s per hour. 

*The thermal conductivity is assumed constant between 0° F. and 
400° F. 


MELTING POINT 


(The temperature at which the substance changes from solid 
to liquid form) 

°Fahr. 


Carbon dioxide 

Ice. 

Wax . 

Tin. 

Lead. 

Copper . 

^ Cast Iron .. .. 

- Steel . 

•-Platinum. 


— loS (ioS° below zero) 

32 

142 to 154 
446 
620 

1943 ' ^ 

1922 to 2075 . ■ ■ 

2372 to 2532 ' - - 

'3110103227 ■ 


















70 


UTILIZATION OF GAS APPLIANCE COURSE 


HEAT OF COMBUSTION 


B. T. U/s per pound. 

Hydrogen (to water vapor at 212° F.). 60000 

Carbon (charcoal) . 14500 

Carbon Monoxide. 43^5 

Methane (Marsh gas). 23600 

Sulphur. 4000 


GLOSSARY OF TECHNICAL TERMS 

CHEMICAL 

Atom.— The smallest portion into which an element may be 
divided. 

Compound.— A combination of elements which may not be 
separated into its component parts or split up into other 
substances by physical means (filtering, sifting, etc.), and 
v/hich possesses properties differing from those of its com¬ 
ponent parts. (See Mixture.) 

Element.— A substance that may not by any known process be 
divided or split up into other substances. 

Mixture.— A combination of substances which may be sepa¬ 
rated into its component parts by physical means (filtering, 
sifting, crystallization, etc.). (See Compound.) 

Molecule.— The smallest portion of a substance that can exist 
by itself. 

ELECTRICAL 

Ampere.— The unit of electrical current or quantity of elec¬ 
tricity, practically defined as that current which will cause 
the electro-deposition of silver at the rate of .001118 grams 
per second. 

The Kilo-Watt is 1,000 watts. 

The Kilo-Watt Hour is the practical unit of energy, and is 
represented by the expenditure of i Kilo-watt of power 
for one hour, or at the rate of 10 Kilo-watts per hour for 







UTILIZATION OF GAS APPLIANCE COURSE 71 

i-io hour, or i-io Kilo-watt per hour for lo hours, etc. 
Thus the Kilo-watt hours are obtained by multiplying the 
Kilo-watts by the time in hours. 

Ohm.— The unit of electrical resistance, represented by the 
resistance at o° centigrade of a column of pure mercury 
106.3 centimeters long, of uniform cross-section and weigh¬ 
ing 14.4521 grams. 

Volt. —The unit of electrical pressure of electromotive force 

1000 

(E.M.F.) practically represented by - of the electro- 

1434 

motive force of a standard Clark cell at a temperature if 

15° c. 

An electromotive force of i volt will send i ampere 
through a resistance of one ohm, thus when any two 
quantities are known, the third may be calculated by the 
formula: 

volts E 

-= amperes or — = C 

resistance R 

Where E = E. M. F. 

R= Resistance. 

C = Current. 

The Watt is the unit of electrical power and is represented 
by one ampere at an E. M. F. of i volt. The watts in 
in any circuit at any instant are obtained by multiplying 
the volts by the amperes, that is, amperes X volts = watts 
or C X E = W, where 
W = Watts. 

E = E. M. F. 

C = Current. 

The Watt-Hour is the unit of electrical energy and is repre¬ 
sented by the expenditure of one watt during one hour. It 
I 

is-of the kilo-watt-hour. 


1000 





72 


UTILIZATION OF GAS APPLIANCE COURSE 


RELATING TO GAS MANUFACTURE, 
DISTRIBUTION AND UTILIZATION 

MANUFACTURE 

Carbonization. —The process of driving off the volatile con¬ 
stituents of coal. 

CoAi. Gas. —Gas produced by heating coal in a closed vessel to 
a temperature sufficient to drive off its volatile constituents, 
leaving nothing but carbon (in the form of coke) and ash 
behind. 

Gas Candle-Power is the luminous intensity (measured in 
candle-power) in a horizontal direction from an Argand 
burner or (less frequently from the flat side of an open 
tip burner) consuming 5 cu. ft. of gas per hour, and is 
obtained by direct comparison with a standard of known 
intensity. It is an indication of the amount of illumi- 
nants in the gas. 

Heating (or Calorific) Value signifies the heating power of 
the gas expressed in British Thermal Units (B. T. U.’s). 
One B. T. U. is the quantity of heat required to raise one 
pound of water one degree Fahrenheit in temperature 
(from 62° F. to 63° F.), and is obtained by burning a 
quantity of gas in such a manner that all the heat pro¬ 
duced is transmitted to a definite weight of water. By 
measuring the increase in temperature of the water, multi¬ 
plying by the weight of the water and dividing by the 
quantity of gas consumed, the B. T. U.’s per cu. ft. are 
obtained. 

Illuminants. —Gaseous or liquid constituents of illuminating 
gas which emit light when burned in an open flame burner. 

Water Gas. —A gas made by decomposing steam by means of 
incandescent carbon (coal or coke). The oxygen com¬ 
bines with the carbon, forming carbon monoxide (CO), 
and liberating hydrogen, the resulting gas being mainly a 
mixture of the two. Water gas is usually carburetted by 
the addition of oils which break up and form illuminants. 


UTILIZATION OF GAS APPLIANCE COURSE 


73 


DISTRIBUTION 

Governor (Pressure).— A device for regulating (or keeping 
uniform) the gas pressure at the outlet; while the pres¬ 
sure at the inlet varies. Also used for reducing from a 
high to a low pressure. 

Pressure (expressed in inches or tenths of an inch) signifies 
the height of a column of water supported by the pressure 
of the gas, and is obtained by measuring the difference 
between the levels of a quantity of water in a U-shaped 
tube, one end of which is connected to the gas supply, the 
other being open to the air. 

Specific Gravity signifies the relative weights of equal vol¬ 
umes of gas and air obtained by weighing equal volumes 
of each at a given pressure, and dividing the weight of the 
gas by that of the air. 

UTILIZATION 

Air Shutter. —In a Bunsen burner, the means provided for 
varying the size of air-ports. 

Bunsen Burner. —A gas burner in which a portion of the air 
required for combustion is mixed with the gas before the 
latter enters the flame. 

Bunsen Flame.— The flame produced in a Bunsen burner. 

Entrainment.— The sucking in of primary air (by the ve¬ 
locity of the gas) through the air-ports of a Bunsen burner. 

Mixing Chamber. —The portion of a Bunsen burner provided 
for the mixing of gas and primary air. 

Nozzle or Tip. —In a Bunsen burner, the hole through which 
the mixture of gas and primary air is delivered to the 
flame. 

Open Flame Burner (also called flat flame, yellow flame, 
white flame, luminous flame, etc.). A gas burner in 
which gas is delivered directly into the flame without pre¬ 
vious admixture of air. 

Primary Air. —The air mixed with the gas before entering the 
flame in a Bunsen burner. 


74 


UTILIZATION OF GAS APPLIANCE COURSE 


Orifice. —In a Bunsen burner, the hole through which the gas 
issues. 

Secondary Air.— Air from the outside of the flame in a 
Bunsen burner. 

PHYSICAL 

RELATING TO HEAT. 

British Thermal Unit. (See under Gas Manufacture.) 

Latent Heat. The heat required to change a substance from 
one form into another (solid to liquid, liquid to vapor, etc.) 
without change of temperature. 

Specific Heat.— The ratio of the quantity of heat required 
to increase the temperature of one pound of a substance i° 
Fahr. to that required to increase the temperature of i 
pound of water from 62° to 63° Fahr. 

Thermal Conductivity.— Capacity for conducting heat, rep¬ 
resented by the quantity of heat per hour conducted by a 
substance through a plate i ft. square and i inch thick, 
with a difference of temperature of 1° F. between oppo¬ 
site sides. 

RELATING TO LIGHT. 

Brightness (of a luminous or illuminated surface).—The 
candle-power given by the surface at a given angle di¬ 
vided by the apparent (not surface) area. The apparent 
area of a 6-inch ball globe viewed from any angle is that 
of a circular area 6 inches in diameter. The apparent 
area of a flat surface 6 inches square depends upon the 
angle at which it is viewed and is equal to the surface 
area multiplied by the sine* of the angle between the 
surface and the line of vision. From a point directly 
facing the surface the apparent area is the same as the 
surface area—6 X 6 = 36 sq. inches. Viewed edgewise, 

* The sine (abbreviated sin of an angle is a mathematical constant of 

the angle. Tables containing sines of all angles will be found in any 

Engineer’s Handbook. 



UTILIZATION OF GAS APPLIANCE COURSE 


75 


the apparent area is o; at 30" from the edgewise position 
the apparent area is 6 X 6 X (sine 30° = .5) = 18 sq. 
inches. 

Candle-Power.— The unit of solid-angular intensity of emis¬ 
sion of light, represented by one lumen per steradian. 

Foot-Candle.— The unit of intensity of illumination, repre¬ 
sented by one lumen per sq. ft. 

Incandescence.— The state of a body in which it becomes 
luminous by virtue of its temperature. 

Lumen.— The unit of quantity of light, represented by ten 
times the amount of light given out in a horizontal direc¬ 
tion toward a surface 3 inches square and 30 inches dis¬ 
tant from a standard Pentane lamp, corrected to standard 
atmospheric conditions. 

Luminescence.— The state of a body by which it becomes 
luminous by virtue of conditions other than its tempera¬ 
ture. 

Steradian.— The unit by which solid angles are measured 
represented (approximately) by one hundred times the 
solid angle formed at the apex of a pyramid i ft. high and 
.1 ft. square at the base, or at the apex of a cone i ft. 
high and .01 sq. ft. in area at the base. 

RELATING TO MECHANICAL WORK. 

Energy.— The capacity for doing work. A coiled spring, a 
moving body, a combustible solid or gas, or a suspended 
weight possess energy which may be converted into work. 

Foot-Pound (Ft. lb.).—The unit of work, represented by the 
work done in lifting one pound one foot. 

Horse-Power.— A unit of Power or rate at which work is 
done. It is equal to 33,000 ft. lbs. per minute. 

Horse-Power Hour. —The amount of work done by the ex¬ 
penditure of one horse-power for one hour. 

Duty.— The amount of energy in one form obtained from a 
given expenditure of energy or consumption of energy¬ 
carrying substance in another form, as horse-power-hours 
per lb. of steam, candle-power-hours per watt or per cu. 
ft. of gas, etc. 






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